Make sure that the under Application Configuration / Page Generation in the XPages Libraries section that the box to use the ext lib did not get checked. If this is checked and the ext lib is not installed on the server then no pages will load as the application can not load.
Check the specific XPage that you added the custom control to and ensure the the ext lib namespace did not get added to the top.
Finally you can check the console log or error log in the C:\IBM\Domino\data\IBM_TECHNICAL_SUPPORT folder. Sort by date/time and see what error the application / page is throwing.